1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus having an image bearing member, a charging device, a developing device, a transfer device and a cleaning device. In addition, the present invention also relates to a toner for use in the image forming apparatus, and a process cartridge.
2. Discussion of the Background
Electrophotographic image forming methods have been used for various fields. Electrophotographic image forming methods typically include the following processes.
(1) charging the surface of an image bearing member such as photoreceptors (charging process):
(2) irradiating the charged image bearing member with light to form an electrostatic latent image on the image bearing member (light irradiating process);
(3) developing the electrostatic latent image with a developer including a toner to form a toner image on the image bearing member (developing process);
(4) transferring the toner image onto a receiving material fed from a sheet feeding device optionally via an intermediate transfer medium (transfer process);
(5) fixing the toner image to the receiving material upon application of heat and pressure thereto (fixing process); and
(6) removing toner particles remaining on the image bearing member and intermediate transfer medium without being transferred so that the image bearing member and intermediate transfer medium are ready for the next image forming processes (cleaning process).
Image forming apparatuses performing such processes are broadly classified into revolver-type image forming apparatuses in which plural developing devices are arranged around one image bearing member and tandem-type image forming apparatuses which plural image bearing members are serially arranged together with respective developing devices to form respective color images. Revolver-type image forming apparatuses have an advantage of low cost. In contrast, tandem-type image forming apparatuses have an advantage of high speed printing but have a relatively high cost. Recently, tandem-type image forming apparatuses are in the mainstream because of being able to perform high speed printing.
Examples of image forming apparatuses are illustrated in FIGS. 1-3.
Referring to FIG. 1, the image forming apparatus includes an image bearing member 7; a charging device 1 configured to charge the surface of the image bearing member 7; a light irradiating device 2 configured to irradiate the charged image bearing member 7 with imagewise light to form an electrostatic latent image thereon; a developing device 3 configured to develop the electrostatic latent image with a developer (such as one-component developers including a toner and no carrier, and two component developers including a toner and a carrier) to form a toner image on the image bearing member 7; a transfer device 4 configured to transfer the toner image to a sheet of a receiving material fed from a sheet feeding device 9; a cleaning device including a cleaner 6 and an auxiliary cleaner 5, which are configured to remove residual toner particles from the image bearing member 7; and a fixing device 8 configured to fix the toner image on the sheet of the receiving material.
Specific examples of the charging device 1 include short-range chargers, contact chargers and corona chargers, which apply a DC voltage or a DC voltage overlapped with an AC voltage.
Specific examples of the light irradiating device 2 include devices using a laser diode (LD), a light emitting diode (LED), a xenon lamp or the like.
Specific examples of the developing device 3 include one-component developing devices using a one-component developer, and two-component developing devices using a two-component developer.
Specific examples of the transfer device 4 include devices including a transfer belt, a transfer charger, a transfer roller or the like.
Specific examples of the auxiliary cleaner 5 include fur brushes, elastic rollers, rollers covered with a tube, devices having a non-woven cloth or the like. As illustrated in FIG. 2, plural auxiliary cleaners can be provided. In contrast, the image forming apparatus illustrated in FIG. 3 includes no auxiliary cleaner.
Specific examples of the cleaner 6 include cleaning blades which are typically made of a material such as polyurethane rubbers, silicone rubbers, nitrile rubbers and chloroprene rubbers.
Blade cleaning methods have been typically used for conventional image forming apparatuses, and there are many image forming apparatuses having only a cleaning blade. In addition, there are high speed image forming apparatuses having a cleaning device having a blade and a brush located on an upstream side from the blade to prevent a situation in that a large amount of residual toner particles are present at a surface of the image bearing member.
With respect to toner for use in the developer, pulverization toners have been used for conventional image forming apparatus. However, in order to produce high quality images and to improve transferability of toner, recently toners with a small particle diameter and spherical toners have been developed and used. For example, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 01-257857 discloses a spherical toner which is prepared by a wet method such as suspension polymerization and emulsion polymerization. In addition, published examined Japanese patent application No. 04-27897 and JP-A 06-317928 have disclosed spherical toners, which are prepared by subjecting pulverized toners to a heat treatment.
However, small toners and spherical toners tend to have a drawback in that residual toner particles present on the surface of an image bearing member escape through a cleaning blade, resulting in defective cleaning (i.e., resulting in occurrence of a background development problem in that the background of an image is soiled with toner particles). When a high pressure is applied to a cleaning blade to prevent such a problem, an excessive shearing force is applied to a portion of the blade, thereby causing chipping (i.e., omission of a portion) of the cleaning blade, resulting in occurrence of defective cleaning. Alternatively, problems in that the cleaning blade and/or the image bearing member are seriously abraded occur.
When a cleaning blade is seriously abraded, the area of the contact point between the blade and the image bearing member increases, resulting in decrease of the pressure of the cleaning blade to the image bearing member. Therefore, a problem in that small toners or spherical toners cannot be well removed from the image bearing member occurs. Thus, it is hard to well remove a small-size toner or a spherical toner.
In attempting to prevent abrasion of a cleaning blade to which a high pressure is applied, JP-As 2002-244516, 2002-156877, 2002-55580, and 2002-244487 have disclosed techniques in that a lubricant is applied to the surface of the image bearing member to be cleaned by the blade.
In addition, in attempting to prolong the lives of a charging device and an image bearing member, JP-A 2002-229227 discloses a technique in that a non-contact charging device and an image bearing member having a photosensitive layer including a particulate inorganic material are used while applying a lubricant such as zinc stearate to the image bearing member.
Further, JP-A 10-142897 discloses an image forming apparatus in which a lubricant applied to the surface of an image bearing member is smoothed (or large particles of the lubricant is blocked) by a blade at a location between a charging device and a developing device.
However, image forming apparatuses having a lubricant applicator tend to have the following drawbacks.
(1) When an excessive amount of lubricant is applied to an image bearing member, the charging roller contacted with the image bearing member is contaminated, thereby causing defective charging, resulting in formation of abnormal images.
(2) Since the lubricant applied to an image bearing member is mixed with the developer used, the toner in the developer is prevented from being well charged, and thereby electrostatic latent images on the image bearing member cannot be well developed, resulting in formation of abnormal images.(3) Setting of a lubricant applicator in an image forming apparatus increases the size and costs of the apparatus.
Thus, a technique of controlling application of a lubricant to an image bearing member has not yet established. Namely, when a lubricant applicator is provided in an image forming apparatus, various problems are caused. Therefore, it is preferable to provide no lubricant applicator in an image forming apparatus in view of reduction in size and costs of the image forming apparatus.
In attempting to well remove a small toner and/or a spherical toner on an image bearing member with a cleaning blade while preventing abrasion of the cleaning blade and occurrence of the size and const problems, the following proposals have been made.
JP-A 2005-55783 discloses a toner in which plural kinds of same-polarity charge controlling agents are present on the surface of the toner and which includes an external additive, wherein the toner has a volume average particle diameter of not greater than 10 μm, and a shape factor of not greater than 180.
JP-A 2000-112169 discloses a toner in which a particulate auxiliary material is present on the surface of toner particles and which has a shape factor of from 100 to 150.
Spherical toner which is prepared by forming toner particles in an aqueous medium and which has a relatively large average particle diameter tends to be well removed from an image bearing member with a blade because such toner has a small amount of fine toner particles. However, when a small-size spherical toner is used to produce high quality images, toner particles on an image bearing member are not often removed well (i.e., the toner has a low margin for cleanability) because such toner tends to include fine toner particles (having a volume particle diameter of not greater than 4 μm) in an amount of not less than 20% by number.
In attempting to remedy the drawback of the above-mentioned small spherical toner (having a volume average particle diameter (Dv) of from 5.0 to 5.5 μm), a technique in that the content of fine toner particles (having a volume particle diameter of not greater than 4 μm) is reduced to 10% by number or less by classification is proposed. It is described therein that such toner has good blade cleanability. However, performing such a classification operation increases costs and production time of the toner while decreasing yield. Therefore, it is desirable not to perform such a classification operation.
Because of these reasons, a need exists for a technique by which a toner having a volume average particle diameter (Dv) of from 5.0 to 5.5 μm and including fine toner particles having a volume particle diameter of not greater than 4 μm in an amount of not less than 20% by number can be used without causing cleaning problems.